1. Field of the Invention
The present invention provides wireless communications, and more particularly, to a method and apparatus for performing hybrid automatic repeat request (HARQ) using constellation rearrangement and a circular buffer in a wireless communication system.
2. Related Art
Wireless communication has recently been developed to satisfy requirements of high spectral efficiency and reliable communication. However, a fading channel environment and an interference caused by various factors result in a packet error which limits overall system capacity.
Hybrid automatic repeat request (HARQ) is an automatic repeat request (ARQ) protocol combined with forward error correction (FEC) and is strongly considered as one of cutting edge technologies for future reliable communication. The HARQ scheme can largely be classified into two types. One is chase combining (CC) HARQ which is introduced in D. Chase, Code Combining A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets, IEEE Trans. on Commun., Vol. 33, pp. 593-607, May 1985. The other is increment redundancy (IR) HARQ. In the CC HARQ, when a receiver detects an error through cyclic redundancy checking (CRC) while decoding the transmitted packet, the same packet with the same modulation and coding is retransmitted to the receiver. Meanwhile, the IR HARQ retransmits different packets in which parity bits may be manipulated through puncturing or repetition in order to obtain a coding gain.
A multiple input multiple output (MIMO) system is one of promising techniques researched for wireless communication. It is known that spatial diversity significantly increases system capacity without an additional bandwidth by constructing multiple antennas in both of a transmitter and a receiver. Consequently, there is ongoing research on a method of using a transmit diversity gain and a receive diversity gain.
Meanwhile, constellation rearrangement is one of methods for improving an HARQ gain by averaging a reliability difference between constitutional element bits in a quadrature amplitude modulation (QAM) constellation. The constellation rearrangement method may apply for multi-modulation transmission, may apply for a MIMO system using HARQ, and may apply for the support of increment redundancy (IR). The constellation rearrangement introduced in PCT International Unexamined Patent Publication No. WO 2008/018742 filed by the present Applicant provides an additional gain by averaging an intrinsic reliability difference between element bits.
In addition, a circular buffer has one of data structures using a fixed-sized buffer. In a process of storing data in the circular buffer, the data is stored in the circular buffer as if the front and the end of the buffer are in contact with each other. The circular buffer may be used to temporarily store an information bit when the information bit is mapped to a data symbol on the constellation. In particular, the circular buffer may be used for data retransmission when HARQ is performed. After the bit at the end of the buffer is transmitted, if it is required to transmit more information bits, the bits are subject to wrap-around at the end of the buffer and they are transmitted starting from the bit in the front of the buffer.
Accordingly, there is a need to provide a further effective method for performing HARQ by using the constellation rearrangement and the circular buffer.